Radiant electric heaters are well known in which one or more heating elements are supported, for example, on or above a layer of thermal insulation material in a metal dish. Heaters of this kind generally incorporate one or more heating elements selected, for example, from: coiled bare resistance wire; metal ribbon, particularly of corrugated form; infra-red lamp(s).
Such heaters are typically incorporated in cooking, or other heating, appliances having a flat glass-ceramic cooking/heating surface.
Different power levels have been provided by including two or more heating elements in the heater, with a switch arranged to couple the elements into different series and/or parallel configurations in different switch positions.
In designing such heaters various different and possibly conflicting requirements must be accommodated. The resistances of the different heating elements must be chosen so that differing combinations of the elements produce power levels which together form an appropriately distributed sequence between zero and full power. In particular it has been found generally desirable to include a very low power level, of the order of 5 percent of full power. Where possible the intensity of visible light radiation produced by the energised elements at each power setting should be indicative of the power level at that setting; in addition it may be desirable that one or more elements should be energised sufficiently to produce some visible radiation for as many power settings as possible, to provide assurance to the user that the heater is functioning. This is particularly the case for any infra-red lamp heating element that may be included in the heater; the presence of an infra-red lamp element in an appliance typically results in a premium price, so the user is likely to expect that element to be visible in use.